Biology subjects

Power, G. A.

Publications and source records attributed to Power, G. A..

At least 19 recordsLinked to original sources

bioRxiv2026

Does increasing the dose of relaxation strain augment the sarcomerogenic response to isometric resistance exercise?

Increases in serial sarcomere number (SSN) have been suggested to reduce muscle strain injury risk and improve muscle mechanical performance, however the environmental loading conditions optimizing sarcomerogenesis are not well known. It has been suggested that the magnitude of lengthening which fibres are subject to upon deactivation (i.e., the relaxation strain) may provide a stimulus for sarcomerogenesis. However, no study has attempted to amplify this relaxation strain stimulus to directly test this possible mechanism. Sprague-Dawley rats underwent six weeks (twice weekly) of isometric plantar flexion training, either sustained near-maximal contractions, or intermittent burst type contractions (i.e., contract-relax) in which each repetition consisted of eight brief high-frequency trains to magnify the overall incidence of relaxation strain occurrences across the training period. Groups were otherwise matched for peak torque and torque-time integral. Following training, the torque-angle relationship adapted similarly between groups (training x group interaction P=0.249), with active torque increasing ~30-40% across the angles tested (P<0.001). No changes in maximal shortening velocity were observed with training in either group (P=0.577), however peak power output increased by ~19% in Isometric and ~49% in Burst (training effect P=0.002; interaction P=0.152). For SSN, the gastrocnemius showed a divergent training response (Lateral gastrocnemius: Burst +~11%; Isometric +~2%; interaction P=0.001; Medial gastrocnemius: Burst +~9%; Isometric +~3%; interaction P=0.052), while soleus SSN increased similarly between groups (~2-3%) (training effect P=0.029, interaction P=0.543). Overall, amplifying the relaxation strain stimulus may have a positive effect on stimulating sarcomerogenesis, however further research is needed to clarify the extent of this effect, as well as any underpinning mechanisms.

physiology↗

bioRxiv2026

Contraction type dependence of serial sarcomere addition following resistance training

Longitudinal muscle growth occurs through the addition of sarcomeres in series, a process termed serial sarcomerogenesis. To investigate contraction mode dependence of sarcomerogenesis in Sprague-Dawley rats (n=36, 14 to18 weeks, ~468g), we compared the effects of isometric, eccentric, and concentric training (4 to 6 weeks; 3 times/week) on morphological and mechanical adaptations of the plantar flexors. Isometric training was performed at a joint angle corresponding to the plateau nearing the descending limb of the force length relationship, eccentric contractions began at a shorter muscle length and lengthened to this same joint position, and concentric contractions started at this lengthened position but then actively shortened. Owing to the rapid regulation of serial sarcomere number the 4-and 6-week timepoints were collapsed. While force output increased across all training modes (+16%; p<0.05), longitudinal muscle growth was regulated by contraction type. Both isometric and eccentric training were highly effective at increasing serial sarcomere number when optimal loading conditions (high forces at longer lengths) are provided (+7-9%; p<0.05). Concentric training, even when initiated at long lengths with high forces was not an effective stimulus for longitudinal remodelling (+2%; p>0.05). Therefore, these findings identify the mechanical loading environment associated with isometric and eccentric training as a key driver of serial sarcomerogenesis.

physiology↗

bioRxiv2025

Changes in serial sarcomere number of five hindlimb muscles across adult aging in rats

IntroductionAging is associated with a reduction in muscle fascicle length (FL), which contributes to sarcopenia: the age-related loss of muscle mass and function. Studies on rodents have confirmed this reduced FL is driven by a loss of sarcomeres aligned in series (serial sarcomere number; SSN) along a muscle. However, studies on rodents have focused primarily on rat plantar flexor SSN at two aging timepoints, leaving an incomplete view of age-related changes in SSN. Hence, this study investigated SSN as a contributor to the age-related loss of muscle mass in five hindlimb muscles across four aging timepoints in rats. MethodsThe soleus, medial gastrocnemius (MG), plantaris, tibialis anterior (TA), and vastus lateralis (VL) were obtained from 5 young (8 months), 5 middle-aged (20 months), 5 old (32 months), and 5 very old (36 months) male F344BN rats. After fixation of muscles in formalin and digestion in nitric acid, fascicles were teased out end-to-end to measure FL. SSN was determined by dividing FL by sarcomere length measured via laser diffraction. Muscle wet weight, anatomical cross-sectional area (ACSA), and physiological cross-sectional area (PCSA) were also determined for insight on age-related losses of whole-muscle mass and in-parallel muscle morphology. ResultsAge-related SSN loss was apparent after middle age for all muscles, with the plantaris showing the smallest (8%) and the VL the greatest (21%) loss. The MG and VL appeared to plateau in their SSN loss by 32 months, while the soleus and TA underwent continued loss from 32 to 36 months. In all muscles, SSN loss evidently contributed in part to the loss of muscle mass, alongside losses of contractile tissue in parallel (indicated by ACSA and PCSA). ConclusionAs SSN is closely tied to biomechanical function, these findings present SSN as a distinct target for improving muscle performance in older adults.

physiology↗

bioRxiv2024

Submaximal eccentric training during immobilization does not prevent serial sarcomere loss or impairments in mechanical function in old or young rats

The age-related loss of muscle mass is partly driven by a reduction in serial sarcomere number (SSN), and further SSN loss occurs during immobilization. SSN is associated with optimal force and power production and muscle passive tension, thus immobilization-induced SSN loss is especially a concern for older individuals who are often subjected to forced muscle disuse with illness and injury. We previously showed that submaximal eccentric resistance training increased SSN and improved muscle function in old rats. The present study investigated whether this training could prevent the losses of SSN and function when performed intermittently during immobilization. 10 old (32 months) and 10 young (8 months) rats underwent unilateral casting of the plantar flexors in a shortened position for 2 weeks. Thrice weekly, casts were removed for isokinetic eccentric resistance training. Pre- and post-training we assessed in-vivo maximum isometric torque at ankle angles corresponding to stretched and neutral muscle lengths, the passive torque-angle relationship, and isotonic power. The soleus and medial gastrocnemius were harvested for SSN measurements, with the untrained leg as a control. In old and young rats, muscles of the casted leg had lower muscle wet weights (-20-40%), physiological cross-sectional area (-16-20%), and SSN (-7-29%) than the control leg. Furthermore, maximum isometric torque (-37-46%) and isotonic power (-{approx}70%) decreased, and passive torque increased (+{approx}400%) from pre- to post-training for both age groups. Thus, irrespective of age, submaximal eccentric resistance training 3 days/week was ineffective for preventing the losses of muscle contractile tissue and mechanical function during casting.

physiology↗

bioRxiv2024

Investigating the temporal effects of ovarian failure on single muscle fibre contractility using a chemically-induced ovarian failure model in mice

We investigated the effects of chemically-induced ovarian failure on single muscle fibre contractility of the soleus and extensor digitorum longus (EDL) muscles throughout ovarian failure, thereby mimicking the menopausal transition into late-stage menopause: [(D60;peri-menopause), (D120;onset of menopause), (D134;early-onset menopause), (D176;late-stage menopause)]. We used 4-vinylcyclohexene diepoxide (VCD) to induce ovarian failure in sexually-mature female mice. At D120 and D176, mice with VCD-induced ovarian failure produced higher force as compared with controls (p<0.05). On D134, however, VCD had lower force production compared with controls (p<0.05). The cross-sectional area of the soleus fibres from the VCD group was larger at D120 compared with controls (p<0.05), but not at any other time point (p>0.05). As well, at D120-D176, the proportion of Type II fibres relative to Type I increased for the soleus, but not the EDL. No differences in rate of force redevelopment (Ktr) was observed for the soleus (p>0.05), while calcium sensitivity increased by late-stage menopause (p<0.05). There were no differences in force, cross-sectional area, stiffness, Ktr, or calcium sensitivity between groups for the EDL (p>0.05). Muscle contractility across the peri-menopausal transition into late-stage menopause is both muscle and phase-dependent, emphasizing the complexity of changing hormones throughout the lifespan on muscle contractile function.

physiology↗

bioRxiv2024

Submaximal eccentric resistance training increases serial sarcomere number and improves dynamic muscle performance in old rats

The age-related loss of muscle mass is partly accounted for by the loss of sarcomeres in series. Serial sarcomere number (SSN) influences muscle mechanical function including the force-length and force-velocity-power relationships, and the age-related loss of SSN contributes to declining performance. Resistance training biased to active lengthening (eccentric) contractions increases SSN in young muscle, however, we showed maximal eccentric training in old rats did not alter SSN and further worsened performance. A submaximal eccentric training stimulus may be more conducive to adaptation for aged muscle. Therefore, the purpose of this study was to assess whether submaximal eccentric training can increase SSN and improve mechanical function in old rats. Twelve 32-month-old male F344/BN rats completed 4 weeks of submaximal (60% maximum) isokinetic eccentric plantar-flexion training 3 days/week. Pre- and post-training, we assessed in-vivo maximum isometric torque at a stretched and neutral ankle angle, the passive torque-angle relationship, and the isotonic torque-angular velocity-power relationship. The soleus and MG were harvested for SSN measurements via laser diffraction, with the untrained leg as a control. SSN increased 11% and 8%, and muscle wet weight increased 14% and 13% in the soleus and MG, respectively. Maximum isometric torque and shortening velocity were unaltered, but there was a shift towards longer muscle lengths for the optimal angle of torque production, a 42% reduction of passive torque, and 23% increase in peak isotonic power. Eccentric training at 60% maximum was beneficial for aged muscle, increasing SSN and muscle mass, reducing muscle passive tension, and improving dynamic performance. New & NoteworthyO_LIFour weeks of submaximal (60% of maximum) eccentric resistance training increased serial sarcomere number and muscle mass in old rats C_LIO_LIIncreased serial sarcomere number corresponded to broadening of the torque-angle relationship following training, which contributed to increased peak isotonic power C_LIO_LISubmaximal eccentric training also led to a large reduction in passive torque throughout the range of motion C_LI

physiology↗

bioRxiv2024

Residual force enhancement is not altered while force depression is amplified at the cellular level in old age

Residual force enhancement (rFE) and residual force depression (rFD) are history-dependent properties of muscle which refer to increased and decreased isometric force following a lengthening or shortening contraction, respectively. The history-dependence of force is greater in older than younger adults when assessed at the joint level. However, it is unclear whether this amplification of the history-dependence of force in old age is owing to cellular mechanisms or a consequence of age-related remodeling of muscle architecture. Single muscle fibres from the psoas major of old and young F344BN rats were dissected and chemically permeabilized. Single muscle fibres were mounted between a force transducer and length controller, then maximally activated (pCa 4.5). To assess rFD, fibers were actively shortened from 3.1 to 2.5{micro}m at both a slow (0.15Lo/s) and fast (0.6Lo/s) speed, with a fixed-end isometric reference contraction at 2.5{micro}m. To assess rFE, fibers were activated and stretched at 0.3Lo/s from a sarcomere length of 2.2 to 2.5{micro}m, and 2.7 to 3.0{micro}m, and compared to fixed-end isometric reference contractions at 2.5 and 3.0{micro}m, respectively. Isometric force was {approx}19% lower in old as compared with young (p<0.001). Upon normalizing to fibre cross-sectional area, there was no age-related difference in specific force (p>0.05). rFD was {approx}33% greater in old as compared with young (p<0.05), while rFE did not differ between groups (p>0.05). rFD is amplified in old age at the cellular level, while rFE appears to be unchanged, thus previously reported age-related modification of rFE occurs upstream from the cellular level.

physiology↗

bioRxiv2024

Age-related differences in the loss and recovery of serial sarcomere number following disuse atrophy in rats

BackgroundOlder adults exhibit a slower recovery of muscle mass following disuse atrophy than young adults. At a smaller scale, muscle fibre cross-sectional area (i.e., sarcomeres in parallel) exhibits this same pattern. Less is known, however, about age-related differences in the recovery of muscle fibre length, driven by increases in serial sarcomere number (SSN), following disuse. The purpose of this study was to investigate age-related differences in SSN adaptations and muscle mechanical function during and following muscle immobilization. We hypothesized that old rats would experience a similar magnitude of SSN loss during immobilization, however, take longer to recover SSN than young following cast removal, which would limit the recovery of muscle mechanical function. MethodsWe casted the plantar flexors of young (8 months) and old (32 months) male rats in a shortened position for 2 weeks, and assessed recovery during 4 weeks of voluntary ambulation. Following sacrifice, legs were fixed in formalin for measurement of soleus wet weight and SSN with the un-casted soleus acting as a control. Ultrasonographic measurements of pennation angle (PA) and muscle thickness (MT) were also conducted weekly. In-vivo active and passive torque-angle relationships were constructed pre-cast, post-cast, and following 4 weeks of recovery. ResultsFrom pre- to post-cast, young and old rats experienced similar decreases in SSN (-20%, P<0.001), muscle wet weight (-25%, P<0.001), MT (-30%), PA (-15%, P<0.001), and maximum isometric torque (-40%, P<0.001), but there was a greater increase in passive torque in old (+180%, P<0.001) compared to young rats (+68%, P=0.006). Following cast removal, young exhibited quicker recovery of SSN, PA, and MT than old, but SSN recovered sooner than PA and MT in both young and old. Muscle wet weight recovered 90% and active torque fully recovered in young rats, whereas in old these remained unrecovered at 75% and 72%, respectively. ConclusionsThis study showed that old rats retain a better ability to recover longitudinal compared to parallel muscle morphology following cast removal, making SSN a highly adaptable, appealing mechanism for restoration of functional capacity following disuse in elderly populations.

physiology↗

bioRxiv2024

Residual force enhancement decreases when scaling from the single muscle fibre to joint level in humans

Residual force enhancement (rFE), defined as increased isometric force following active lengthening compared to a fixed-end isometric contraction at the same muscle length and level of activation, is present across all scales of muscle. While rFE is always present at the cellular level, often rFE non-responders are observed during joint-level voluntary contractions. We compared rFE between the joint level and single fibre level (vastus lateralis biopsies) in 16 young males. In-vivo voluntary knee-extensor rFE was measured by comparing steady-state isometric torque between a stretch-hold (maximal activation at 150{degrees}, stretch to 70{degrees}, hold) and a fixed-end isometric contraction, with ultrasonographic recording of vastus lateralis fascicle length (FL). Fixed-end contractions were performed at 67.5{degrees}, 70{degrees}, 72.5{degrees}, and 75{degrees}; the joint angle that most closely matched FL of the stretch-hold contractions isometric steady-state was used to calculate rFE. The starting and ending FLs of the stretch-hold contraction were expressed as % optimal FL, determined via torque-angle relationship. In single fibre experiments, the starting and ending fibre lengths were matched relative to optimal length determined from in-vivo testing, yielding an average sarcomere excursion of [~]2.2-3.4{micro}m. There was a greater magnitude of rFE at the single fibre ([~]20%) than joint level ([~]5%) (P=0.004), with non-responders only observed at the joint level. By comparing rFE across scales within the same participants, we show the development of the rFE non-responder phenomenon is upstream of rFEs cellular mechanisms, with rFE only lost rather than gained when scaling from single fibres to the joint level.

physiology↗

bioRxiv2024

No effect of menstrual cycle phase on eccentric exercise-induced neuromuscular impairments and the magnitude of the repeated bout effect

Neuromuscular function is impaired following an unaccustomed bout of eccentric exercise, however, through the repeated bout effect (RBE) the muscle is protected from damage following a subsequent bout of eccentric exercise. As a result of unaccustomed eccentric contractions, structural muscle damage occurs in both sexes. However, the inflammatory response may be mitigated in females due to estradiol, thereby attenuating the secondary effects of muscle damage and potentially limiting the magnitude of the RBE. We investigated the relationship between menstrual cycle phase and oral contraceptive use on neuromuscular impairments following the first bout of exercise, and the magnitude of the RBE. Thirteen female participants performed two bouts of 150 maximal eccentric voluntary contractions of the elbow flexors four weeks apart. Normally menstruating females participated during the late follicular phase (day 10-14) of their menstrual cycle, as determined through cycle tracking, when estradiol is near peak, and progesterone is lower. Oral contraceptive users were tested on their placebo pill days (lower estradiol). Neuromuscular function was assessed for Bout 1 before the eccentric protocol and then again 48 hours following, and this was repeated 4 weeks later for Bout 2. Eccentric exercise-induced muscle weakness and soreness did not differ between groups following Bout 1 (p=0.885), and the magnitude of the RBE (p<0.05) was similar between groups (p=0.995). Females in the late follicular phase (classified as high estradiol) and females on combined oral contraceptives (low estradiol) had similar impairments in neuromuscular function following the first bout of eccentric exercise, and a similar RBE.

physiology↗

bioRxiv2023

Age-related blunting of serial sarcomerogenesis and mechanical adaptations following 2 weeks of maximal eccentric resistance training

BackgroundDuring natural aging, muscles atrophy, which is partly accounted for by a loss of sarcomeres in series. Serial sarcomere number (SSN) is associated with aspects of muscle mechanical function including the force-length and force-velocity-power relationships; hence, the age-related loss of SSN contributes to declining performance. Training emphasizing muscle lengthening (eccentric) contractions increases SSN in young healthy rodents. However, the ability for eccentric training to increase SSN and improve mechanical function in old age is unknown. MethodsTen young (9 months) and 11 old (33 months) Fisher344/BN F1 rats completed 4 weeks of unilateral isokinetic eccentric plantar flexion training 3 days/week. Pre- and post-training, the plantar flexors were assessed for maximum tetanic torque (ankle angles of 70{degrees} and 90{degrees}), the torque-frequency relationship (stimulation frequencies of 1-100 Hz), the passive torque-angle relationship (ankle angles of 110-70{degrees}), and the torque-angular velocity-power relationship (isotonic loads of 10%-80% maximum). Following post-training testing, rats were sacrificed, and the soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) were harvested for SSN assessment by measuring sarcomere lengths with laser diffraction, with the untrained leg used as a control. ResultsIn the untrained leg/pre-training, old rats had lower SSN in the soleus (-9%), LG (-7%), and MG (-14%), lower maximum torque (-27 to -42%), power (-63%), and shortening velocity (-35%), and greater passive torque (+62 to +191%) than young. Young rats showed increased SSN from the untrained to the trained soleus and MG. In contrast, old rats had no change in soleus SSN between legs and experienced SSN loss in the LG. Pre- to post-training, young rats saw modest improvements in isometric mechanical function, including a 13% increase in maximum torque at 90{degrees} and 4-11% increases in 10-60 Hz torque. Old rats, however, had reductions in maximum torque (-35%), shortening velocity (-46%), and power (-63%), and increased passive torque (+24 to +51%) from pre- to post-training. ConclusionsEccentric training induced serial sarcomerogenesis and improved mechanical function in young rats, while old rats exhibited dysfunctional remodeling that led to impairments in muscle mechanical performance following training.

physiology↗

bioRxiv2023

Effects of intensified training with insufficient recovery on joint level and single muscle fibre mechanical function: The role of myofibrillar Ca2+ sensitivity

Intense exercise training with insufficient recovery is associated with reductions in neuromuscular performance. However, it is unclear how single muscle fibre mechanical function and myofibrillar Ca2+ sensitivity contribute to these impairments. We investigated the effects of overload training on joint-level neuromuscular performance and cellular-level mechanical function. Fourteen athletes (4 female, 10 male) underwent a 3-week intensified training protocol consisting of [~]140% of their regular training hours with three additional high-intensity training sessions per week. Neuromuscular performance of the knee extensors was assessed via maximum voluntary contraction (MVC) force, electrically evoked twitch contractions, and a force-frequency relationship. Muscle biopsies were taken from the vastus lateralis to assess single fibre mechanical function. Neither MVC force nor twitch parameters were altered following intensified training (all p>0.05), but a rightward shift in the force-frequency curve was observed with a 6-27% reduction in force at low-frequencies (5-20Hz, all p<0.05). In single fibres, maximal force output was not reduced following intensified training, but there was a rightward shift in the force-pCa curve driven by a 6% reduction in Ca2+ sensitivity as indicated by a lower pCa50 value (i.e., higher [Ca2+]) across fibre types (Pre=6.477{+/-}0.157, Post=6.088{+/-}0.480, p<0.05). These data indicate intensified training leads to impaired Ca2+ sensitivity at the single fibre level, which in part explains impaired neuromuscular function at the joint level during lower frequencies of activation. This is an important consideration for athletes, as performance is often assessed at maximal levels of activation, and these underlying impairments in force generation may be less obvious. New & NoteworthyIntense exercise training with insufficient recovery leads to impaired muscle contractile performance. These impairments often manifest at lower frequencies of muscle stimulation, termed prolonged low-frequency force depression. Impaired myofibrillar calcium sensitivity has been suggested as a potential mechanism of prolonged low-frequency force depression. Our work shows that impaired calcium sensitivity of single muscle fibres coincided with joint level prolonged low-frequency force depression after intense exercise training with insufficient recovery.

physiology↗

bioRxiv2023

The Effect of Gradual Ovarian Failure on Dynamic Muscle Function and the Role of High Intensity Interval Training on Mitigating Impairments

Skeletal muscle function is impaired in menopause and exercise may mitigate tshis decline. We used the VCD model of menopause to investigate the effects of gradual ovarian failure on skeletal muscle contractile function and whether high intensity interval training (HIIT) can mitigate impairments. Sexually mature female CD-1 mice were assigned to one of three groups: control (n=5), VCD-sedentary (n=5), or VCD-training (n=5). Following ovarian failure, the VCD-training group underwent 8 weeks of uphill HIIT. Mice were sacrificed 8 weeks after ovarian failure, representing late menopause. Single muscle fibres from the soleus (SOL) and extensor digitorum longus (EDL) muscles were dissected, chemically permeabilized, and mechanically tested. Single muscle fibres were maximally activated (pCa 4.5) then isotonic load clamps were performed to calculate force-velocity-power curves. Absolute force and peak power were 31% and 32% lower in VCD-sedentary fibres compared to control fibres, respectively, in both SOL and EDL muscles. Despite reductions in absolute force and therefore lighter relative loads imposed during the isotonic contractions in VCD-sedentary fibres, there were no concomitant increases in contractile velocity. HIIT was partially effective at mitigating power loss (22% higher peak power in VCD-training compared to VCD-sedentary), but only in fast-type SOL fibres. These findings indicate that ovarian failure impairs dynamic contractile function - likely through a combination of lower force-generating capacity and slower shortening velocity - and that HIIT may be insufficient to completely counteract the deleterious effects of menopause at the cellular level. New & NoteworthyO_LIReductions in circulating ovarian hormones impair static muscle contractile performance, but less is known about dynamic properties like power. C_LIO_LITypically, rodent models of menopause completely remove the ovaries and fail to mimic the prolonged and complex hormonal transition that includes a retention in ovarian androgen production. C_LIO_LIUsing an ovary intact VCD model of ovarian failure, we found that single fibre power was impaired compared with controls in both SOL and EDL fibres. C_LIO_LIOur uphill high intensity interval training program was partially sufficient to reverse power loss, but only in fast-type SOL fibres. C_LIO_LIImpairments in muscle power following ovarian failure are likely driven by a combination of decreased muscle size and force-generating capacity. C_LI

physiology↗

bioRxiv2023

Ultrasonographic measurements of fascicle length overestimate adaptations in serial sarcomere number

Ultrasound-derived measurements of muscle fascicle length (FL) are often used to infer increases (chronic stretch or training) or decreases (muscle disuse or aging) in serial sarcomere number (SSN). Whether FL adaptations measured via ultrasound can truly approximate SSN adaptations has not been investigated. We casted the right hindlimb of 15 male Sprague-Dawley rats in a dorsiflexed position (i.e., stretched the plantar flexors) for 2 weeks, with the left hindlimb serving as a control. Ultrasound images of the soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) were obtained with the ankle at 90{degrees} and full dorsiflexion for both hindlimbs pre and post-cast. Following post-cast ultrasound measurements, legs were fixed in formalin with the ankle at 90{degrees}, then muscles were dissected, and fascicles were teased out for measurement of sarcomere lengths via laser diffraction and calculation of SSN. Ultrasound detected an 11% increase in soleus FL, a 12% decrease in LG FL, and an 8-11% increase in MG FL for proximal fascicles and at full dorsiflexion. These adaptations were partly reflected by SSN adaptations, with a 6% greater soleus SSN in the casted leg than the un-casted leg, but no SSN differences for the gastrocnemii. Weak relationships were observed between ultrasonographic measurements of FL and measurements of FL and SSN from dissected fascicles. Our results showed that ultrasound-derived FL measurements can overestimate an increase in SSN by [~]5%. Future studies should be cautious when concluding a large magnitude of sarcomerogenesis from ultrasound-derived FL measurements, and may consider applying a correction factor. Key Points SummaryO_LIMeasurements of muscle fascicle length via ultrasound are often used to infer changes in serial sarcomere number, such as increases following chronic stretch or resistance training, and decreases with aging or muscle disuse C_LIO_LIThe present study used a rat model of casting the plantar flexor muscles in a stretched position to investigate directly whether ultrasound-derived fascicle length can accurately detect adaptations in serial sarcomere number C_LIO_LIUltrasound detected an [~]11% increase in soleus fascicle length, but measurements on dissected fascicles showed the actual increase in serial sarcomere number was only [~]6%; therefore, measurements of ultrasound-derived fascicle length can overestimate serial sarcomere number adaptations by as much as 5% C_LI

physiology↗

bioRxiv2023

Effects of VCD-induced ovarian failure on single muscle fiber contractility in a mouse model of menopause

ObjectiveMenopause is associated with impairments in muscle contractile function. The temporal and mechanistic basis of this dysfunction are not known. Using a mouse model of menopause we identified how gradual ovarian failure affects single muscle fiber contractility. Study designMice were injected with VCD over 15 days and ovarian failure developed over 120 days. Mice were then sacrificed and slow-type soleus (SOL) and fast-type extensor digitorum longus (EDL) muscles were dissected and chemically permeabilized for mechanical testing. Main outcome measuresMuscle fiber contractility was assessed via: force, rate of force redevelopment, instantaneous stiffness, and calcium sensitivity across three relative force levels (pCa10,pCa50,pCa90). ResultsPeak force and cross-sectional area (CSA) of the SOL were [~]33% and [~]24% greater in the VCD group as compared with controls (P<0.05), respectively, with no differences in force produced by the EDL fibers across groups (P>0.05). Upon normalizing force to CSA there were no differences across groups (P>0.05). Rate of force development was [~]33% faster for SOL in the VCD group compared to control. Ca2+ sensitivity did not differ between groups for either muscle at pCa50 (P>0.05). In the VCD group, Ca2+ sensitivity was higher for EDL, but lower for SOL at pCa10 and pCa90 (P<0.05), respectively. ConclusionsIn our mouse model of menopause, alterations to muscle contractility were much less evident as compared with ovariectomized models. This divergence across models highlights the importance of better approximating the natural trajectory of menopause during and after the transitional phase of ovarian failure on neuromuscular function.

physiology↗

bioRxiv2022

An increase in serial sarcomere number induced via weighted downhill running improves work loop performance in the rat soleus

Increased serial sarcomere number (SSN) has been observed in rats via downhill running training due to the emphasis on active lengthening contractions; however, little is known about the influence on dynamic contractile function. Therefore, we employed 4 weeks of weighted downhill running training in rats, then assessed soleus SSN and work loop performance. We hypothesized trained rats would produce greater net work output during faster, higher-strain work loops due to a greater SSN. Thirty-one Sprague-Dawley rats were assigned to a control or training group. Weight was added during downhill running via a custom-made vest, progressing from 5-15% body mass. Following sacrifice, the soleus was dissected, and a force-length relationship was constructed. Work loops (active shortening followed by passive lengthening) were then performed about optimal muscle length (LO) at 1.5-3-Hz cycle frequencies and 1-7-mm strains to assess net work output. Muscles were then fixed in formalin at LO. Fascicle lengths and sarcomere lengths were measured and used to calculate SSN. Intramuscular collagen content and crosslinking were quantified via a hydroxyproline content and pepsin-solubility assay. Trained rats had longer fascicle lengths (+13%), greater SSN (+8%), greater specific active forces (+50%), and lower passive forces (-45-62%) than controls (P<0.05). There were no differences in collagen parameters (P>0.05). Net work output was greater (+101-424%) in trained than control rats for the 1.5-Hz loops at 1, 3, and 5-mm strains (P<0.05) and showed relationships with fascicle length (R2=0.14-0.24, P<0.05). These results suggest training-induced longitudinal muscle growth may improve dynamic performance.

physiology↗

bioRxiv2021

Perception of effort during an isometric contraction is influenced by prior muscle lengthening or shortening

PurposeFollowing a shortening or lengthening muscle contraction, the torque produced in the isometric steady state is distinctly lower (residual torque depression; rTD) or higher (residual torque enhancement; rTE), respectively, compared to a purely isometric contraction at the same final muscle length and level of activation. This is referred to as the history dependence of force. When matching a given torque level, there is greater muscle activation (electromyography; EMG) following shortening and less activation following lengthening. Owing to these differences in neuromuscular activation, it is unclear whether perception of effort is altered by the history dependence of force. MethodsExperiment 1 tested whether perception of effort differed between the rTD and rTE state when torque was matched. Experiment 2 tested whether perception of effort differed between the rTD and rTE state when EMG was matched. Finally, experiment 3 tested whether EMG differed between the rTD and rTE state when perception of effort was matched. ResultsWhen torque was matched, both EMG and perception of effort were higher in the rTD compared to rTE state. When EMG was matched, torque was lower in the rTD compared to rTE state while perception of effort did not differ between the two states. When perception of effort was matched, torque was lower in the rTD compared to rTE state and EMG did not differ between the two states. ConclusionThe combined results from these experiments indicate that the history dependence of force alters ones perception of effort, dependent on the level of motor command.

neuroscience↗

bioRxiv2020

Age-related reductions in the number of serial sarcomeres contribute to shorter fascicle lengths but not elevated passive tension

We investigated age-related changes to fascicle length (FL), sarcomere length (SL), and serial sarcomere number (SSN), and how this affects passive force. Following mechanical testing to determine passive force, the medial gastrocnemius muscle of young (n=9) and old (n=8) Fisher 344BN hybrid rats was chemically fixed at the optimal muscle length for force production; individual fascicles were dissected for length measurement, and laser diffraction was used to assess SL. Old rats had [~]14% shorter FL than young, which was driven by a [~]10% reduction in SSN, with no difference in SL ([~]4%). Passive force was greater in the old compared to young rats at long muscle lengths. Shorter FL and reduced SSN in the old rats could not entirely explain increased passive forces for absolute length changes, owing to a slight reduction in SL in old, resulting in similar SL at long muscle lengths. Summary StatementThis study sought to explain the increased passive tension observed for muscles of older individuals owing to age-related changes to muscle architecture.

physiology↗